In a closed-loop emission control apparatus for internal combustion engines, a signal representing the concentration of an exhaust composition is generated by detecting the particular composition and modulated in amplitude in accordance with a predetermined control characteristic to provide a control signal which is fed back to an air-fuel mixing and proportioning device, whereby the air-fuel ratio of the mixture is controlled at a preset value. A catalytic converter is provided to simultaneously convert the noxious components present in the emissions into harmless products when the ratio is controlled at the preset value. However, the mixture ratio is also under the influence of various factors which include the operating characteristics of the closed control loop that vary from one vehicle to another and with time over the period of use of the apparatus, and external conditions that affect the performance of the engine. These factors combined will cause the control point of the system to drift from the optimum point where the catalytic converter provides maximum conversion efficiency.
To determine whether the air-fuel ratio is actually controlled at the optimum point for the catalytic converter, it has been proposed to provide an additional sensor at the downstream side of the catalytic converter to generate a compensating signal. However, the use of additional sensor will add to the complexity and cost of the apparatus.